In the meantime I have contacted Dave Moir
(for many years head glassblower at Strathearn) about this issue (with some electronic detour via Derek).
Dave reports that at Strathearn for the standard spoke weights bubbles were certainly not planned. It was a case of producing as many as possible in as short a time as possible.
He also mentions that the colours they used had quite different melting temperatures - white was particularly soft (low melting point) whereas red was hard (high melting temperature). Now - in the examples from my weight, in both cases the bubbles formed over white/grey areas surrounded by red/orange. Would the temperature of the hot plate (where canes were set up) be sufficient for the white to be softened? Also - would glass shrink when softened - or would some effect of the surrounding colours also be required to produce small indentations (for the later bubbles)?